Line a 8 x 8-inch (20 x 20cm) baking pan (preferably glass) with parchment paper, leaving overhang on two sides.

Whisk together all-purpose flour, rice flour, rosemary, and salt; set aside.

Combine the butter, ½ cup (100g) sugar, and grapefruit zest in the bowl of a stand mixer and beat with paddle attachment on medium-high speed until light and fluffy, 2 to 3 minutes, scraping down bowl as needed.

Add reserved flour mixture. Beat on low speed until crumbly, then increase speed to medium and beat until well combined, scraping down bowl as needed, about 2 minutes more.

Transfer dough to prepared pan, then use an offset spatula to spread and press dough evenly into pan, smoothing top. Chill until firm, 1 hour.

Preheat oven to 375°F (190°C). Prick shortbread all over with a fork. Sprinkle evenly with remaining 1 tablespoon sugar, then sprinkle flaky salt over.

Bake until golden brown around edges and evenly golden on surface, 30 to 35 minutes.

Remove from oven and immediately cut shortbread (still in pan) into four 2”-wide strips lengthwise.

Rotate pan 90° and make eight 1-inch (2.5cm)-wide cuts perpendicular to the first ones to make 32 rectangular cookies.

Let sit 10 minutes, then run a thin spatula around edges so that cookies don’t adhere to pan as they cool. Let cool completely in pan.

Using parchment, lift cookies out of pan and retrace cut marks if necessary to separate cookies.

A note on making these ahead: Cookies can be baked, cut, and cooled 1 week in advance. Transfer (on parchment) to an airtight container and store at room temperature, or freeze for up to 3 months.